The present invention relates to fluid controllers of the type used to control the flow of fluid from a source of pressurized fluid to a fluid pressure actuated device, such as a steering cylinder for steering a vehicle. More particularly, the present invention relates to such a fluid controller having at least two different modes of operation, in terms of the relationship between the manual input to the fluid controller and the rate of fluid flow out of the controller.
Although the present invention may be used in connection with fluid controllers of many types, and having various constructions and applications, it is especially advantageous when used in conjunction with a full-fluid-linked steering controller, for use on a vehicle of primarily the "off highway" type, and will be described in connection therewith.
A conventional fluid controller of the type to which the present invention relates includes a housing which defines various fluid ports, and further includes a fluid meter, a valve means defining a main fluid path, and an arrangement for imparting follow-up movement to the valve means, in response to the flow of fluid through the fluid meter. The flow through the controller valve means is directly proportional to the areas of the variable flow control orifices in the main fluid path. As is well know to those skilled in the art, the area of each flow control orifice is, in turn, typically proportional to the rate at which the steering wheel is rotated.
A typical application for a full-fluid-linked steering controller of the type to which the present invention relates would be a vehicle such as is used on a work site, and such a vehicle would be used in one of two operating modes. First, the vehicle may be operated in a "roading" mode, i.e., it is driven on the road, at normal roading speeds, in order to reach a work site. Second, the vehicle may be operated in a "working" mode, at the work site and is performing work related operations, such as moving a pile of dirt, etc., during which the vehicle is moving at relatively slow speeds.
The roading and working modes of operation described above present very different steering requirements, as is now well know to those skilled in the art. When roading the vehicle, a relatively low gain rate would be desirable, whereas, when operating in the working mode, a relatively high gain rate would be desirable. As used herein, the term "gain rate" refers to the rate of change of steered wheel position for a given amount of steering input (such as, but not limited to, rotation of a vehicle steering wheel). With a conventional full-fluid-linked steering controller, however, the gain rate is actually a constant, and as a result, the amount of steering motion by the vehicle operator while roading is typically acceptable, but the amount of steering motion required at the work site, over the course of a typical workday, can cause excessive operator fatigue.
One approach to providing a steering system which gives the operator separate reading and working modes of operation has been to provide the vehicle operator with a steering wheel for use when the vehicle needs to be in the roading mode, and with a joy stick for use when the vehicle needs to be in the working mode. The steering wheel gives the operator somewhat the same feel as driving an automobile, which is desirable for the roading mode, while the joy stick may be used to provide relatively large steering changes with relatively little operator input (a large gain rate), which is desirable for use in the working mode.
Unfortunately, the provision of a steering wheel/joy stick system adds substantially to the overall expense and complication of the system, in terms of the hardware involved, and also results in substantial complication and expense in order to coordinate the portion of the system operated by the steering wheel with the portion of the system operated by the joy stick.